1. Field of the Invention
The present invention relates to a novel polymeric electrolyte and an electrochemical device comprising the same. More particularly, the present invention is concerned with a novel hybrid polymeric electrolyte which is formed of a closed-cell cellular polymer foam impregnated with a non-aqueous electrolytic liquid and which comprises a plurality of closed cells defined by cell walls constituting a continuous solid-phase matrix impregnated with the non-aqueous electrolytic liquid to form a continuous solid-phase domain, wherein each of the plurality of closed cells is substantially filled with a non-aqueous electrolytic liquid to form a plurality of liquid phase domains which are dispersed in the above-mentioned continuous solid-phase domain. The present invention is also concerned with a non-aqueous electrochemical device, such as a non-aqueous battery or parts for a battery (e.g., an electrode), which comprises the above-mentioned hybrid polymeric electrolyte. The hybrid polymeric electrolyte of the present invention has not only high ionic conductivity and high mechanical strength, but also has the ability to prevent the non-aqueous electrolytic liquid contained therein from leakage, so that the electrolyte of the present invention can be advantageously used in various non-aqueous electrochemical devices. That is, the non-aqueous electrochemical device comprising the hybrid polymeric electrolyte of the present invention exhibits not only excellent electrochemical performance, but also has high ability to retain an electrolytic liquid therein, so that the electrochemical device-has excellent safety and high reliability in practical use thereof.
2. Prior Art
Recently, for reducing the size and weight of portable equipment, such as pocket telephones and personal computers, it has been demanded to provide a battery having high energy density. As a battery for meeting such a demand, lithium batteries have been developed and commercially produced. These lithium batteries are wet type batteries which contain a porous polyolefin separator having through-holes, wherein the pores of the separator are filled with a non-aqueous electrolytic solution which is used as a medium for transporting ions between the positive and negative electrodes. However, wet type batteries have problems in that a leakage of the non-aqueous electrolytic solution is likely to occur and it is difficult to realize light weight batteries.
By contrast, solid type batteries produced using a solid electrolyte are free from a leakage of an electrolytic solution, differing from the above-mentioned wet type batteries using a non-aqueous electrolytic solution as such. Therefore, it is expected that a solid electrolyte not only provides a battery having improved reliability and safety, but is also advantageous in that both the lamination of a solid electrolyte with electrodes, and the packaging of the resultant laminate to form a battery can be easily performed, and the thickness and weight of a battery can be reduced. As materials for such a solid electrolyte, ion-conductive ceramics and polymeric solid electrolytes have been proposed. Of these materials, the ion-conductive ceramic is disadvantageous in that a ceramic is brittle, so that it is difficult to produce a laminate structure of a ceramic with electrodes. By contrast, the polymeric solid electrolyte inherently has good workability and flexibility, so that the polymeric solid electrolyte is advantageous in that when it is used in an electrochemical device, such as a battery, it is easy to produce a laminate structure of the polymeric solid electrolyte with electrodes, and also the polymeric solid electrolyte is capable of allowing the configuration of its interface with the electrodes to change in accordance with the volumetric change of the electrodes caused by the occlusion and release of ions by the electrodes.
As such a polymeric solid electrolyte, an alkali metal salt complex of polyethylene oxide was proposed by Wright in British Polymer Journal, vol. 7, p. 319 (1975). Since then, researches on various materials for polymeric solid electrolytes have been energetically conducted. Examples of such materials include polyalkylene ethers, such as polyethylene glycol and polypropylene oxide, polyacrylonitrile, polyphosphazene, polyvinylidene fluoride and polysiloxane.
Generally, these polymeric solid electrolytes are provided in the form of solid solutions of an electrolyte and a polymer, and are known as dry type polymeric electrolytes. Further, gelled polymeric solid electrolytes are known which are obtained by incorporating an electrolyte and a solvent for the electrolyte into a polymer matrix, wherein the solvent is intended for increasing the dissociation of the electrolyte and promoting the molecular movement of the polymer (see, for example, Japanese Patent Application Laid-Open Specification No. 57-143356). As a method for introducing an electrolyte and a solvent for the electrolyte into a polymer matrix, there have been known, for example, a method in which a uniform solution of a polymer, an electrolyte and a solvent for the electrolyte is cast into a film (see, for example, U.S. Pat. No. 5,296,318), and a method in which a mixture of a polymer and a plasticizer is cast into a film; the plasticizer is extracted from the film to obtain a polymer matrix, and the polymer matrix is impregnated with an electrolytic solution obtained by dissolving an electrolyte in a solvent for the electrolyte, or, alternatively, after the casting of the film, the plasticizer in the film is replaced by an electrolytic solution. In the latter method, the use of the plasticizer in addition to the solvent for the electrolyte is intended to facilitate the swelling of the polymer.
The polymer used for production of the above-mentioned gelled polymeric solid electrolyte is a polymer which can easily form a uniform solution with an electrolyte and a solvent therefor. Therefore, for example, when a vinylidene fluoride polymer is used as the polymer, the obtained gelled polymeric solid electrolyte easily melts at 85.degree. C. to 110.degree. C. to exhibit flowability, so that there is a danger that a battery using such a gelled polymeric solid electrolyte suffers short circuiting at high temperatures, thus posing a safety problem. For solving this problem, a hybrid polymeric electrolyte has been proposed which is obtained by a method in which a mixture of a polymer, a plasticizer and a polymerizable vinyl monomer is cast into a film; the polymerizable vinyl monomer is cross-linked; the plasticizer is extracted from the film to obtain a polymer matrix; and the polymer matrix is impregnated with. an electrolytic solution (see U.S. Pat. No. 5,429,891). However, this method is disadvantageous not only in that the practice of this method is cumbersome, but also in that the polymerizable vinyl monomer is electrochemically unstable, and the plasticizer and the polymerizable vinyl monomer are susceptive to side reactions at the time of the cross-linking. Therefore, this hybrid polymeric electrolyte cannot be practically used for a battery.
As polymeric solid electrolytes having improved mechanical strength, there have been proposed a composite polymeric solid electrolyte obtained by introducing an ion-conductive polymer, such as polyethylene oxide, into a porous olefin polymer having through-holes (Japanese Patent Application Laid-Open Specification No. 63-102104); a polymeric solid electrolyte obtained by casting into a film a mixture of an ion-conductive polymer latex and an ion non-conductive polymer latex (Japanese Patent Application Laid-Open Specification No. 4-325990); and a polymeric solid electrolyte having a structure such that ceramic particles are dispersed in a polymer (Japanese Patent Application Laid-Open Specification No. 2-276164).
On the other hand, a miniature battery using, as a separator, a polyurethane foam porous material has been proposed (German Democratic Republic Patent No. 241159). However, the separator illustratively disclosed in this prior art is a separator having through-holes. Further, since the separator has a urethane linkage, the separator has a problem in that it is electrochemically unstable. Also, a primary battery using as an electrolyte a sulfonated polystyrene foam has been proposed (Japanese Patent Application Laid-Open Specification No. 2-94261). However, a sulfonated polystyrene foam has problems in that it is difficult to impregnate it with a non-aqueous electrolytic solution, and that a sulfonated polystyrene has water absorptivity and it is difficult to remove water therefrom, so that a sulfonated polystyrene cannot be used for a non-aqueous battery.
These polymeric solid electrolytes have a problem in that the ion-conductivity of them is small as compared to that of an electrolytic liquid. Therefore, a battery produced using such a polymeric solid electrolyte has defects such that it has low charge/discharge current density and has high resistance. For this reason, development of a polymeric solid electrolyte having high ion-conductivity has been desired. The ion-conductivity of a dry type polymeric solid electrolyte, such as a solid solution of an electrolyte and polyethylene oxide, is very low, so that, when a battery using such a dry type polymeric solid electrolyte is operated at room temperature, the current density obtained is limited to an extremely low level. Further, a gelled polymeric solid electrolyte containing a plasticizer has high ion-conductivity as compared to a dry type polymeric solid electrolyte. However, when the amount of the plasticizer is increased in order to increase the ion conductivity, problems arise such that the mechanical strength becomes low and it becomes difficult to control the thickness of the polymeric solid electrolyte.
On the other hand, with respect to the lithium ion secondary battery currently in use, which contains a porous polyolefin separator wherein the pores of the separator are filled with an electrolytic solution (see, for example, Examined Japanese Patent Application Publication No. 59-37292), there is a problem in that a polyolefin has extremely low ion permeability, so that the ion conductivity of the polyolefin separator (having pores simply filled with an electrolytic solution) is low as compared to that of the electrolytic solution. Further, since the electrolytic solution filled in the pores of the polyolefin separator can easily flow out of the separator, it is necessary to package the battery structure in a very strong metallic casing having a large thickness.